The survey has been conducted following a multi-stage stratified sampling design, where First Stage Units (FSUs) are UFS (Urban Frame Survey) blocks in urban areas and villages in rural areas as per Census 2011 (updated by removing those villages which are urbanized and included in latest UFS) except for rural areas Kerala, where Panchayat wards have been taken as FSUs. The ultimate stage units (USUs) are households for both the sectors. Both the FSUs and USUs have been selected with Simple Random Sampling Without Replacement (SRSWOR). In the case of large FSUs, large villages/UFS blocks are notionally divided into smaller units of more or less equal size, known as sub-units. 8 sample households were canvassed within an FSU.
The survey period has been divided into four sub-rounds of three months' duration each as follows:
sub-round 1: January - March 2025
sub-round 2: April - June 2025
sub-round 3: July - September 2025
sub-round 4: October - December 2025
The sampling frame for urban sector is the list of Urban Frame Survey (UFS) blocks as per latest Urban Frame Survey and for rural sector, it is the list of villages as per Census 2011 updated by removing those villages which are urbanized and included in latest UFS (till the time of sample selection).Sometimes, with a view to ensure uniformity in the size of FSUs and operational convenience, large villages/UFS blocks are notionally divided into smaller units of more or less equal size, known as sub-units depending on a pre-defined criteria based on population in the village or number of households in the UFS block.
STRATIFICATION OF FSUs
The primary geographical unit, called basic stratum within a state/UT for rural and urban sectors is a district.
Rural Sector:
(i) A Special Stratum comprising of all the uninhabited villages as per Census 2011 is formed at all-India level.
(ii) From the remaining villages, two more strata are formed in each basic stratum. i.e., in each district:
(a) Stratum 1: Comprising of the villages within 5 Kms from the district headquarter or from a city/town with more than 5 lakh population. This stratum is not formed if there are less than 50 such villages in the stratum.
(b) Stratum 2: Rest of the villages.
Urban Sector:
Two or more strata will be formed in urban areas of each basic stratum with the following criteria:
(i) Each million plus city as per census 2011 will be considered as a separate stratum.
(ii) Rest of the urban areas of the district will constitute another stratum.
SUB-STRATIFICATION OF FSUS
In the rural sector, three groups of villages are formed within each stratum, except special rural stratum at all-India level, based on the following criteria:
Group Population of the village (as per Census 2011)
- all villages (Panchayat wards for Kerala) with Census 2011 population less than 250
- all villages (Panchayat wards for Kerala) with Census 2011 population more than or equal to 250 but less than 500
- remaining villages
Further, the sample size for a particular rural stratum is distributed among these 3 groups in proportion to population. If the number of FSUs in a particular group is very small or sufficient number of samples is not allocated, no sub-stratum will be formed. In the same line, urban sub-stratification will be formed.
SAMPLE SIZE
Total 17,636 FSUs have been allocated for the central sample at all-India level for the health survey. The total number of FSUs have been equally distributed among four sub-rounds, each having a sample of 4409 FSUs. A minimum of 16 FSUs (8 each for the rural and urban sector) were allocated to each State/ UT.
SELECTION OF FSUS WITHIN STRATUM/SUB-STRATUM
The required number of FSUs from each of the stratum / sub-stratum is selected by SRSWOR, independently for each of the sub-rounds.
